It doesn't because mating cousins is likely the effect of a skewed population,not the cause.

(im not a historian so this is speculation)

I imagine that early humans had divided roles, and roles assigned to males involved more danger. (bearing in mind that the state of medicine meant that relatively minor incidents by our standards could lead to infection and death in theirs.)

So populations start out 50/50,but skew badly once men age enough to take on dangerous work.

This leads to men having multiple partners and the scenario mentioned in the sibling comment.